http://www.legislation.gov.uk/ukpga/2017/9/pdfs/ukpga_201700...
very simple and unambiguous
saying that, the UK parliament can declare things to be unlawful retroactively, so it's possible
just at "bernie can still win" levels of possibility
I think everyone believes the Article 50 declaration was properly done. But proper isn't politics. Parliament can say whatever it wants. If on March 30th, 2019 Britain says it's in the EU, a disagreeing EU member would need to begin enforcing customs, tariffs, et cetera. If nobody, or nobody who matters, does that the rule goes unenforced.
It reminds me of an oversight in the North Dakota Constitution, which did not contain a clause required for accession [1]. (It was later added.) Laws are just codified conventions; the EU has an enforcement problem in that it cannot enforce, only its member states can.
[1] http://newsfeed.time.com/2011/07/14/because-of-constitution-...
I'm not quite sure how that would work, given article 50 was issued after an act of parliament.